In fill-level sensors operating according to the FMCW or pulse-transit time method, electromagnetic or acoustic waves are emitted in the direction of a feed material surface. Subsequently, a sensor records the echo signals reflected by the feed material, the container internals and the container itself, and from these derives the respective fill level.
Fill-level sensors operating according to a transit time method may, for example, comprise sensors which determine the distance to a feed material surface with the use of ultrasound, radar, guided radar, FMCW radar, pulse-burst radar, CW radar or laser. Furthermore, all the sensors that emit a wave and subsequently receive and evaluate the components of said wave that are reflected by a feed material surface may be defined as fill-level sensors that operate according to a transit time method.
In the use of acoustic or optical waves the signal generated by the fill-level measuring device generally speaking propagates freely in the direction of the feed material surface to be measured. In the case of devices that use radar waves for measuring the feed material surface, both free propagation in the direction of the medium to be measured, and propagation in the interior of a waveguide that guides the radar waves from the fill-level measuring device to the medium can be considered. In the case of devices according to the principle of the guided microwave the high-frequency signals are guided along a waveguide towards the medium.
On the surface of the medium to be measured, some of the incoming signals are reflected, and after a corresponding transit time return to the fill-level measuring device. The non-reflected signal components enter the medium and propagate in it, according to the physical characteristics of the medium, in the direction of the container bottom. These signals are reflected by the container bottom and, after passing through the medium and the overlaid atmosphere, return to the fill-level measuring device.
Moreover, it is desirable to use fill-level measuring devices for separating layer measuring and also for emulsion measuring.
In the field of separating layer measuring, usually two different liquids are placed in a container. Because of the different densities of these at least two liquids, they de-mix in the container, provided adequately settled conditions are present, in other words provided the liquids are not prevented from de-mixing by an agitator or by some other equipment.
DE 10 2007 061 574 A1 describes a method for measuring emulsions by combining a measurement on the basis of a guided microwave with a capacitive measurement.